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Cellular Injury During Ischemia
The Ischemic Penumbra
 Within
the ischemic cerebrovascular bed, there are two major zones
of injury: the core ischemic zone and the "ischemic penumbra"
(the term generally used to define ischemic but still viable
cerebral tissue).
In the core zone, which is an area of severe
ischemia (blood flow below 10% to 25%), the loss of inadequate
supply of oxygen and glucose results in rapid depletion of
energy stores. Severe ischemia can result in necrosis
of neurons and also of supporting cellular elements (glial
cells) within the severely ischemic area.
Brain cells within the penumbra, a rim of mild
to moderately ischemic tissue lying between tissue that is
normally perfused and the area in which infarction is evolving,
may remain viable for several hours. That is because
the penumbral zone is supplied with blood by collateral arteries
anastomosing with branches of the occluded vascular tree (see
inset). However, even cells in this region will die
if reperfusion is not established during the early hours since
collateral circulation is inadequate to maintain the neuronal
demand for oxygen and glucose indefinitely.
In this example, the ischemic penumbra is shown
as a rim of tissue surrounding the severely ischemic core
lying within the vascular territory of the pre-Rolandic branch
of the left middle cerebral artery. The Rolandic artery
is occluded by a thromboembolus. The extent of the penumbra
varies directly with the number and patency of collateral
arteries.
The penumbra is where pharmacologic interventions
are most likely to be effective. However, it may also
be possible to salvage cells within the severely ischemic
core zone. Although severe ischemia kills selectively
vulnerable neurons, glial cells may be spared if blood flow
is restored early. Therefore, timely recanalization
of the occluded vessel should theoretically restore perfusion
in both the penumbra and in the severely ischemic core.
Partial recanalization should markedly reduce the size of
the penumbra as well.
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